Supernova Nucleosynthesis Andrea Kulier Princeton University, Department of Astrophysical Sciences November 25, 2009
Outline o Overview o Core-Collapse Supernova Nucleosynthesis o Explosive Nucleosynthesis o Carbon Burning o Neon Burning o Oxygen Burning o Silicon Burning o Observations of Nucleosynthesis in Core-Collapse Supernovae o Nucleosynthesis in Simulations of Core-Collapse Supernovae o SNIa Nucleosynthesis o Nucleosynthesis in Simulations of SNIa Supernovae
Overview of Supernova Nucleosynthesis o changes the content of the ISM o produces heavy elements o nuclear decay of 56 Co, produced during supernova nucleosynthesis, powers the light from supernova remnants o observations of gamma rays emitted by supernova nucleosynthesis can provide important constraints on theoretical models o currently the deepest probe of supernova ejecta (gravitational waves would be deeper) Radioactive Isotopes Synthesized in Supernovae Hernanz, 2001
Core-Collapse Supernovae o nucleosynthesis in core-collapse supernovae occurs in the form of explosive burning when the shock wave passes through layers of unburned material, igniting and accelerating it o although reaction systems are complicated, yields can be predicted with use of some simplifying approximations: o Nuclear Statistical Equilibrium: for sufficiently high temperature and density, strong and electromagnetic thermonuclear reaction rates are rapid enough to achieve equilibrium within the timescale of the supernova o Quasi-Equilibrium: for temperatures in the range of 3-6 GK, although there is no global equilibrium, many nuclei are in local equilibrium with their neighbors
Carbon Burning o calculations of explosive carbon burning for constant temperature done in Arnett and Truran 1969 o took constant density of 5 10 5 g cm -3 o nucleosynthesis during carbon burning does not sensitively depend on the density o initial composition of equal parts 12 C and 16 O o reactions in carbon burning that produce light particles: 12 C ( 12 C, α) 20 Ne 12 C ( 12 C, p) 23 Na 12 C ( 12 C, n) 23 Mg o these light particles react much faster than the original 12 C + 12 C reaction o most significant reactions: 16 O (α, γ) 20 Ne 23 Na (p, α) 20 Ne 23 Na (p, γ) 24 Mg 20 Ne (α, γ) 24 Mg Nuclear reaction flow diagram for carbon burning at T = 1.2 10 9 K with little carbon depletion
Carbon Burning Nuclear reaction flow diagram for carbon burning at T = 1.2 10 9 K at half carbon depletion o neutron excess = (total neutrons total protons)/total nucleons o the number of free neutrons, protons, and alpha particles is small o an increase in proton absorbers, particularly 23 Na, results in a decrease in protons and a constant number of alpha particles through the 23 Na (p, α) 20 Ne reaction o neutron number is kept high by 12 C ( 12 C, n) 23 Mg o when 12 C depletion becomes significant, the number of neutrons and alpha particles also begins to decrease
Carbon Burning
Carbon Burning o at T = 0.8 10 9 K, the gross behavior is similar to that at T = 1.2 10 9 K
Carbon Burning
Neon Burning o neon and carbon burning have many similarities o the main reaction is 20 Ne(γ, α) 16 O o protons and neutrons are produced by (α, n) and (α, p) reactions o behavior of abundances of neutrons, protons, and alpha particles is similar to that in carbon burning o creates similar products to carbon burning, although 22 Ne and 23 Na are reduced in abundance o carbon burning produces about three times more energy than neon burning
Oxygen Burning o quasi-equilibrium behavior o less sensitive to reaction rates than carbon burning and more dependent on binding energies o heavy elements (Z > 28) begin to photodissociate at the higher temperatures required for oxygen burning o Woosley et al. 1973 did an analysis of explosive oxygen and silicon burning as an improvement on Truran and Arnett 1970, which was on explosive oxygen burning o initial composition for oxygen burning was about half 16 O and half 12 C o peak density was taken as 2 10 5 g cm -3 o peak temperature was taken as 3.6 10 9 K
Oxygen Burning o calculation by Woosley succeeded in reproducing major abundances from 28 Si to 42 Ca, as well as the correct solar abundances of 46 Ti and 50 Cr o calculations also agreed well with Truran and Arnett o driving reactions: 16 O ( 16 O, α) 28 Si 16 O ( 16 O, p) 31 P 16 O ( 16 O, n) 31 S 16 O ( 16 O, d) 30 P 16 O (γ, α) 12 C o higher temperature and lower density favor this channel
o silicon burning also shows quasi-equilibrium behavior Silicon Burning o if 16 O is depleted before freezeout, nuclear evolution will become characterized by photodisintegration rearrangement reactions o produces a sizable amount of iron peak nuclei o overall silicon burning process: 28 Si ( 28 Si, 7α) 56 Ni o Woosley et al. start with initial conditions of temperature 4.7 10 9 K and density 2 10 5 g cm -3 o continue the silicon burning where the oxygen burning ended o end up with good approximation of solar abundances o extreme silicon burning will merge with the e-process and approach nuclear statistical equilibrium
Observations of Nucleosynthesis in Core- Collapse Supernovae o Type II supernova SN 1987A o light curve showed decline consistent with 56 Co o gamma ray lines from 56 Co detected at 847 and 1238 kev Credit: X-ray: NASA/CXC/PSU/ S.Park & D.Burrows.Optical: NASA/STScI/CfA/P. Challis o confirmed by balloon-borne experiments o line profiles of 56 Co imply that the supernova explosion was not spherically symmetric Credit: NASA/CXC/MIT/U Mass Amherst/ M.D.Stage et al. o 57 Co decay detected o type II supernova Cassiopeia A o obtained abundance estimates of S, Ar, Ca, Fe, and Ne, and upper limits for H, He, N, Mg, and C o displayed expected quasi-equilibrium pattern expected of oxygen burning
Simulations of Core-Collapse Supernovae o although we do not know the precise mechanism by which a core-collapse supernova proceeds, nucleosynthesis in a core collapse supernova can be computed by artificially simulating a supernova explosion o injection of momentum through a piston that moves inward during the infall prior to the explosion and outward during the explosion o does not allow material in the ejecta to slow down and collapse back onto the star o underestimates fallback, leading to an overestimate of the amount of heavy elements, such as 56 Ni o thermal bomb injection of thermal energy into the Fe core in a way such that the ejecta attains a kinetic energy of ~10 51 erg o designed to incorporate the energy increase in the convective region
Thermonuclear Supernovae (SNIa) o probable mechanism for SnIa: delayed detonation o burning is subsonic (deflagration) in the inner core where densities are large and supersonic (detonation) in the outer core o pure detonation at densities larger than ~10 7 g cm -3 would cause the entire star to be incinerated to Ni o deflagration overproduces some neutronized isotopes, such as 54 Fe, 54 Cr, and 58 Ni o 56 Ni lines (158, 750, 812 kev) prominent during first few days o 158 kev line is the most interesting to discriminate between models o almost undetectable in pure deflagration models o strong in pure detonation models o ratio between fluxes of 56 Co 847 kev line (200 days after maximum) and 158 kev line (at maximum) o provides information about the ratio between 56 Ni in the ejecta and the external layers
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